Horn Antenna for the 21cm Neutral-Hydrogen Line

Abstract : Van de Hulst predicted the existence of the radio emission of neutral hydrogen at 21 cm and in 1951 Ewen and Purcell succeeded in detecting this emission with the horn antenna shown above. In this post we want to describe the design and the construction of a low cost horn antenna with the aim to detect and measure this emission coming from our milky way.


The hydrogen 21-centimeter line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.4 MHz, which is equivalent to the vacuum wavelength of 21.1 cm in free space. This wavelength falls within the microwave region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light. The study of this radio emission can help us to evaluate the presence of hydrogen in interstellar clouds and to understand the structure of our milky way.


Neutral hydrogen radiates at 1420.4 MHz, and studies have shown that red and blue shifts of up to 2 MHz may be observed. Therefore our goal is to design a horn antenna to receive signals in the range of 1420.4 ± 2 MHz. The antenna is just a length of copper wire cut to l =5.25 cm, about a quarter of the wavelength λ =21.1 cm corresponding to 1420.4 MHz. With these numbers in mind, we have to design a waveguide and horn to efficiently direct desired frequencies toward the antenna (the copper wire).
An open waveguide is not an effective energy radiator (and similarly an effective receiver) due to the impedance mismatch at the mouth. Things improve by widening the walls of the trumpet-shaped guide, thus coupling the impedance between the waveguide and the intrinsic one of free space.


The first component of the system is the waveguide. As we know waveguide is a hollow metal pipe used to carry radio waves. The electromagnetic waves in a (metal-pipe) waveguide may be imagined as travelling down the guide in a zig-zag path, being repeatedly reflected between opposite walls of the guide. For the particular case of rectangular waveguide, it is possible to base an exact analysis on this view and derive the propagation modes and cutoff frequency.
The drawings below shows a rectangular waveguide with the electrical and magnetic field of the main propagation mode (TE10). The direction of the antenna determines the polarization direction of the wave that is picked up, the electrical field is polarized vertically, parallel to the antenna, the intensity is zero on the surface of the metal waveguide and the maximum intensity is reached inside the waveguide in a position depending on the wavelength of the radiation. The pattern of the propagation mode is also characterized by a waveguide wavelength λG .

The cutoff wavelength – the maximum wavelength that can propagate in the waveguide in the direction of the side “a” – is λ = 2a. This is equivalent to saying the waveguide acts as a high pass filter with cutoff frequency of ν = c/λ. The following drawing can help to understand this behavior of the waveguide. In order to minimize dispersion in velocity down the waveguide in the range of interest, a waveguide should be used for frequencies greater than 1.25*ν . Additionally, in order to suppress higher order modes they should be used for frequencies less than 1.9*ν.

Now we have all the data to design our waveguide. Fortunately we can use for this purpose a normal 5lt can (for example a can for oil). The measures of this rectangular container are the following : a = 146 mm, b = 117 mm. We check, referring to the following drawing, that all conditions are verified and we proceed to the calculations for positioning the antenna.

Radio Emission Data
f = 1420,4 MHz – Neutral Hydrogen Emission Frequency
Δf = 2 MHz
f = 1420,4 ± 2 MHz
λ = 21,1 cm – Wavelength
l = λ/4 = 5,25 cm – Quarter Wave Antenna

Waveguide Dimensions
a = 146 mm
b = 117 mm

Waveguide Cutoff Frequency
λc = 2*a = 29,2 cm
fc = c / λc = 1027,4 MHz
f > 1,25*fc = 1283,7 MHz – OK
f < 1,9*fc = 1951,3 MHz – OK

Antenna Positioning
λG = 30,57 cm – Waveguide Wavelength
d = λG/4 = 7,64 cm – Quarter Wave
l = λG*3/4 = 22,92 cm – Waveguide Lenght


A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. An advantage of horn antennas is that since they have no resonant elements, they can operate over a wide range of frequencies, a wide bandwidth. Our antenna is a “Pyramidal horn” : a horn antenna with the horn in the shape of a four-sided pyramid, with a rectangular cross section.
By adopting the measures of similar projects, where a compromise was made between gain and size, we chose the following measures. In addition, our horn antenna will have to connect with the rectangular waveguide described above.


A = 750 mm
B = 600 mm
RE = 700 mm
RH = 700 mm

a = 146 mm
b = 117 mm


With an online application we have determined the antenna gain that turns out to be Gain = 18.16 dB ≈ 18 dB

The beam width can be calculated from the geometry of the antenna. We limit ourselves to reporting the values ​​we obtained from antennas built with similar geometries.

● Gain : 18 dB
● Half Power Beam Width : 20° H-plane, 24° E-plane

These data are however in accordance with the general theory that assigns an angular resolution to an antenna given by the relation:

Δθ ≅ λ/D = 21/75 = 0.28 rad = 16°

The horn antenna is a rather directive antenna, however the dimensions of our antenna are limited and this makes the angle covered rather large. The spatial resolution of our antenna will therefore be about 16° and will not allow us to resolve structures with a spacing of less than 16°. The extension of the radio sources obtained with the antenna will be the convolution of the real extension with the radiation pattern of the antenna : in practice this means an enlargement of the detected dimensions. We report below as an example the radiation pattern of a similar pyramidal horn antenna.


For the construction of our antenna we used a 5lt can as a basis for the waveguide and then we had the 1.2 mm thick raw aluminum sheet cut and bent by a workshop for the construction of the horn.


The upper part was cut from the can, then we cut the edges to leave an integral part of the calculated length of 23 cm. We left the side flaps that will be used for fixing on the aluminum sheet. In the calculated position, a hole was made for a type N panel connector, on which a piece of a thin brass tube was welded which constitutes the actual antenna. An N-SMA adapter is connected to the panel connector, fixed with four screws. The images below show the waveguide.


The horn was assembled from sheet metal cut and folded according to the following drawing.

The waveguide was then fixed to the horn using the edges of the can itself fixed with screws and aluminum adhesive tape. The aluminum adhesive tape was also used to cover the joints between the sheets and with the can so that the inside of the horn is as flat and homogeneous as possible. To increase the rigidity of the structure it may be useful to glue aluminum profiles, for example square shaped, on the four major sides of the horn: in this way the sheets are prevented from deforming due to their own weight.

Antenna Stand

For the mechanical support of our horn antenna, we used a simple wooden bench on which we hinged a wooden base on which the waveguide and the pyramidal horn rest. The inclination is adjusted simply, by raising or lowering the antenna and inserting an adequate support under the base.


The Intenet contains numerous examples of antennas and receivers for the emission of neutral hydrogen at 21 cm. At the following link there is the description of an excellent similar project : probe-the-galaxy-on-a-shoestring-with-this-diy-hydrogen-line-telescope, and this is the related documentation : Hydrogen Line Project Documentation.
A site very rich in information (I would say indispensable ..) is the following DSPIRA

Next Step

This project continues with the construction of the receiver : Low-Noise SDR-Based Receiver for the 21cm Neutral-Hydrogen Line

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